Ring oscillators, such as voltage controlled oscillators (VCOs), are commonly used in communications systems and particularly in phase locked loop (PLL) circuits. In many PLL applications it is often necessary to lock to a range of incoming frequencies. For example, frequency ranges of 2:1 or higher are not uncommon. Tunable ring oscillators may, for example, be used in a PLL to cover the range of required frequencies.
A tunable ring oscillator, in accordance with the teachings described herein, may include one or more delay circuits having a coarse tuning circuitry and a fine tuning circuitry. The coarse tuning circuitry may be used to set one of a minimum time delay or a maximum time delay as a function of a coarse tuning input. The fine tuning circuitry may be used to adjust between the minimum time delay and the maximum time delay as a function of a fine tuning input.